Go outside on a dark, clear night, far away from the glare of city lights, and look up. You will see the glowing band of the Milky Way arching dramatically overhead. It has now been four centuries since Galileo Galilei first turned a telescope toward this awesome sight and noted that the “milk” is actually countless individual stars, too faint to be separated by the naked eye. It took another three centuries for astronomers to convince themselves that the Milky Way is just one of billions of galaxies in the universe.